Polycyclic aromatic hydrocarbons (PAH) are major contaminants in soil and groundwater at a number of sites across the U.S. Many of the PAH are known or suspected carcinogens, yet many are also known to be biodegradable by bacteria and fungi indigenous to native and contaminated soils. Biological processes have been used to remediate PAH-contaminated sites, but removal of the high-molecular weight, carcinogenic species is often incomplete. Since these compounds are poorly soluble in water, their biodegradation of some of the PAH in contaminated systems is limited by factors other than bioavailability. The purpose of this project is to develop approaches by which the factor(s) that limit the degradation of PAH which appear to resist biodegradation can be identified and, eventually, overcome. Such factors can include the absence of significant numbers of microbes able to degrade these compounds; inherently very low rates of degradation of the compound by all organisms, which may be exacerbated by the presence of other PAH; loss of growth substrates for the microorganisms over time and consequent loss of PAH-degrading activity; loss of growth substrates for the microorganisms over time and consequent loss of PAH-degrading activity; and accumulation of products of incomplete metabolism that may inhibit the degradation of one or more PAH. Achievable removals of the carcinogenic PAH may determine whether active bioremediation is an acceptable technology at a given site. In addition, under emerging risk-based approaches to site management such of the PAH contamination is expected to be left in place at many sites. To understand the limits of natural attenuation and potential long-term risks at such sites, it is essential to improve our knowledge of the rates of PAH degradation and the factors that influence these rates. The proposed research will quantify rates of degradation of high-molecular weight PAH by bacteria isolated from a variety of contaminated soils and the effect of other PAH in mixtures on these rates. Then, actual contaminated soils from an industrial site will be treated will be treated in a bench-scale bioreactor to study the degradation of one or more apparently recalcitrant PAH. The factor(s) that govern the degradation of these compounds will be elucidated by: (i) quantifying the organisms in the treated soil that are capable of degrading of these compounds will be elucidated by: (i) quantifying the organisms in the treated soil that are capable of degrading the recalcitrant PAH; (ii) evaluating the effects of adding readily- degraded, naturally occurring compounds known to stimulate PAH degradation; (iii) examining whether the liquid phase in the bioreactor is inhibitory to PAH-degrading bacteria as a result of metabolite accumulation; and (iv) examining the role of bioavailability in the apparent recalcitrance of these compounds. With knowledge of the factor(s) most responsible for limited degradation of the target compound(s), we will then explore methods of stimulating biodegradation either directly in the bioreactor in treated soil removed from the bioreactor.